1. Field of the Invention
The present invention relates to a process for producing an oxide catalyst for use in producing acrylonitrile or methacrylonitrile from propane or isobutane. More particularly, the present invention is concerned with a process for producing an oxide catalyst for use in producing acrylonitrile or methacrylonitrile from propane or isobutane by ammoxidation in the gaseous phase, wherein the oxide catalyst comprises a compound oxide containing molybdenum (Mo), vanadium (V), antimony (Sb) and optionally an element X. The process comprises subjecting a solution or slurry, in water and/or an alcohol, of a raw material mixture comprising a Mo compound, a V compound, an Sb compound and optionally an X compound to a specific oxidation treatment using an oxidizing gas and/or an oxidizing liquid before subjecting the raw material mixture solution or slurry to drying and subsequent calcination. When the oxide catalyst produced by the process of the present invention is used in the production of acrylonitrile or methacrylonitrile, not only can the desired acrylonitrile or methacrylonitrile be produced in high yield and in high space time yield, but also the deterioration of the catalyst is suppressed, so that production of acrylonitrile or methacrylonitrile can be stably performed for a prolonged period of time.
2. Prior Art
Recently, attention has been attracted to a technique for producing acrylonitrile or methacrylonitrile by a gaseous-phase catalytic ammoxidation of propane or isobutane, as a substitute for a gaseous-phase catalytic ammoxidation of propylene or isobutylene, and a number of proposals have been made with respect to a catalyst for use in the ammoxidation of propane or isobutane.
For example, as a catalyst for use in the ammoxidation of propane or isobutane, a compound oxide catalyst containing molybdenum (Mo), vanadium (V), tellurium (Te) and niobium (Nb) is known. Such a compound oxide catalyst is disclosed in, for example, Unexamined Japanese Patent Application Laid-Open Specification Nos. 2-257 (corresponding to U.S. Pat. No. 5,049,692), 5-148212 (corresponding to U.S. Pat. No. 5,231,214), 5-208136 (corresponding to European Patent No. 529,853), 6-227819, 6-285372 (corresponding to U.S. Pat. No. 5,422,328), 7-144132, 7-232071, 8-57319 and 8-141401.
When the above-mentioned catalyst is used for the ammoxidation of propane or isobutane, the yield and space time yield of acrylonitrile or methacrylonitrile {hereinafter, frequently referred to as xe2x80x9c(meth)acrylonitrilexe2x80x9d)} become high. However, this catalyst has a problem in that during the production of the (meth)acrylonitrile, the tellurium is volatilized from the catalyst, thereby leading to a deterioration of the catalyst.
For this reason, as described below, oxide catalysts containing Mo, V and antimony (Sb) and oxide catalysts containing Mo, V, Sb and Nb, each of which contains antimony in place of tellurium which is likely to be volatilized from a catalyst, have been proposed.
For example, a compound oxide catalyst containing Mo, V, Sb and Nb (wherein Nb is an optional component) is disclosed in Unexamined Japanese Patent Application Laid-Open Specification Nos. 9-157241 (corresponding to U.S. Pat. No. 5,750,760) and 10-28862. This catalyst is produced using an aqueous solution containing a molybdenum compound, a vanadium compound, an antimony compound and optionally a niobium compound. The catalyst production process described in those documents comprises mixing a compound of pentavalent vanadium and a compound of trivalent antimony so as to reduce the vanadium, or mixing a compound of hexavalent molybdenum and a compound of trivalent antimony so as to reduce the molybdenum; mixing the resultant reaction mixture with compounds of other component elements to thereby obtain a mixture; and drying the obtained mixture, followed by calcination. Specifically, in these documents, the oxide catalyst is produced by a process comprising heat-aging an aqueous slurry containing a compound of pentavalent vanadium and a compound of trivalent antimony, followed by addition of a molybdenum compound and a niobium compound, thereby obtaining an aqueous mixture; optionally cooling the obtained aqueous mixture; and drying the aqueous mixture, followed by calcination.
When the catalyst produced by the above-mentioned process is used for the production of (meth)acrylonitrile, the desired (meth)acrylonitrile can be produced in a relatively high yield. However, the space time yield of (meth)acrylonitrile [molar amount of (meth)acrylonitrile formed (xcexcmol)/time of contact (contact time) between the raw material and the catalyst (gxc2x7sec/ml)xc3x97catalyst weight (g)] is disadvantageously as low as 0.11 to 0.22 [(xcexcmol/{(gxc2x7sec/ml)xc2x7g}]. Therefore, the productivity of (meth)acrylonitrile per reactor becomes low. Further, when it is attempted to improve the space time yield of (meth)acrylonitrile by employing a high reaction temperature, the selectivity for (meth)acrylonitrile is lowered, so that the yield of (meth)acrylonitrile also is lowered.
Unexamined Japanese Patent Application Laid-Open Specification No. 5-293374 (corresponding to U.S. Pat. No. 5,094,989) describes an oxide catalyst which contains V and Sb as major components, and also contains a small amount of Mo. However, when this catalyst is used in the production of (meth)acrylonitrile, not only is a high reaction temperature needed, but also the yield of (meth)acrylonitrile is low.
New Developments in Selective Oxidation pp. 515-525 (1990) reports the results of the ammoxidation of propane which was performed using a catalyst comprising alumina having supported thereon a compound oxide containing Mo, V and Sb in a molar ratio of 1/0.14/0.71. According to this document, the above-mentioned catalyst has a problem in that not only the selectivity for (meth)acrylonitrile but also the activity to perform the desired conversion are low.
U.S. Pat. No. 4,760,159 describes an oxide catalyst which contains bismuth, vanadium and antimony as major components, and also contains a small amount of molybdenum. However, when this catalyst is used in the production of (meth)acrylonitrile a problem arises not only in that a high reaction temperature is needed, but also in that the yield of (meth)acrylonitrile is low.
As is apparent from the above, with respect to the oxide catalyst comprising Moxe2x80x94Vxe2x80x94Sb or Moxe2x80x94Vxe2x80x94Sbxe2x80x94Nb, the deterioration thereof during the production of (meth)acrylonitrile is suppressed, but not only the conversion-selectivity-based yield (hereinafter referred to simply as xe2x80x9cyieldxe2x80x9d) but also the space time yield of (meth)acrylonitrile is low.
Further, the oxide catalyst comprising Moxe2x80x94Vxe2x80x94Sb or Moxe2x80x94Vxe2x80x94Sbxe2x80x94Nb has the following problem. As described in Applied Catalysis A General, Vol. 157, 143-172 (1997), in the ammoxidation of propane, ammonia is converted to not only acrylonitrile (the desired compound of the ammoxidation) but also to various by-products, such as acetonitrile and hydrocyanic acid as well as nitrogen (an oxidative decomposition product). When the oxide catalyst comprising Moxe2x80x94Vxe2x80x94Sb or Moxe2x80x94Vxe2x80x94Sbxe2x80x94Nb is used for the ammoxidation, the decomposition of ammonia into nitrogen vigorously occurs (that is, the ratio of ammonia decomposed to form nitrogen is high), so that the loss of ammonia is large.
In this situation, the present inventors have made extensive and intensive studies to solve the above-mentioned problems accompanying the prior art. As a result, it has unexpectedly been found that, in the production of an oxide catalyst comprising a compound oxide containing at least molybdenum (Mo), vanadium (V) and antimony (Sb), when a solution or slurry, in water and/or an alcohol, of a raw material mixture comprising at least an Mo compound, a V compound and an Sb compound is subjected to a specific oxidation treatment, and the resultant oxidized raw material mixture solution or slurry is then dried to thereby obtain a dried catalyst precursor, followed by calcination of the dried catalyst precursor, an oxide catalyst can be obtained which is not only insusceptible to deterioration during the production of (meth)acrylonitrile (that is, unlikely to be deactivated), but also effective for producing (meth)acrylonitrile in high yield and high space time yield, and which, therefore, can be advantageously used for effectively, efficiently producing (meth)acrylonitrile for a prolonged period of time.
Further, it has also unexpectedly been found that, when a base-treated oxide catalyst, which can be obtained by treating the above-mentioned oxide catalyst with an aqueous basic solution, is used in the production of acrylonitrile or methacrylonitrile from propane or isobutane by ammoxidation in the gaseous phase, not only can the above-mentioned excellent effects be achieved, but also it becomes possible to suppress the decomposition of ammonia, which is one of the materials for the production of acrylonitrile or methacrylonitrile, into nitrogen, so that the utility of the ammonia can be remarkably enhanced.
The present invention has been completed, based on the above novel findings.
Accordingly, it is a primary object of the present invention to provide a process for producing an excellent oxide-catalyst which is not only insusceptible to deterioration during the production of acrylonitrile or methacrylonitrile, but also effective for stably producing acrylonitrile or methacrylonitrile in high yield and high space time yield, and which, therefore, can be advantageously used for stably conducting an efficient production of acrylonitrile or methacrylonitrile for a prolonged period of time.
It is another object of the present invention to provide a process for producing a base-treated oxide catalyst by treating the above-mentioned oxide catalyst with an aqueous basic solution, wherein the base-treated catalyst not only has the above-mentioned excellent effects, but also is capable of suppressing the decomposition of ammonia, which is one of the materials for the production of acrylonitrile or methacrylonitrile, into nitrogen, so that the utility of the ammonia can be remarkably enhanced.
The foregoing and other objects, and advantages of the present invention will be apparent to those skilled in the art-from the following detailed description taken in connection with the appended claims.
In a primary aspect of the present invention, there is provided a process for producing an oxide catalyst for use in producing acrylonitrile or methacrylonitrile from propane or isobutane by ammoxidation in the gaseous phase, the oxide catalyst comprising a compound oxide represented by the following formula:
xe2x80x83Mo0.1VaSbbXcOnxe2x80x83xe2x80x83(I)
wherein:
X represents at least one element selected from the group consisting of niobium, tungsten, chromium, titanium, tantalum, zirconium, hafnium, manganese, rhenium, iron, ruthenium, cobalt, rhodium, nickel, palladium, platinum, copper, silver, zinc, boron, gallium, indium, germanium, tin, tellurium, phosphorus, lead, bismuth, rare earth elements, and alkaline earth metals; and
a, b, c and n are, respectively, the atomic ratios of vanadium (V), antimony (Sb), X and oxygen (O), relative to molybdenum (Mo),
xe2x80x83wherein
0.1xe2x89xa6axe2x89xa61.0,
0.01xe2x89xa6bxe2x89xa60.6,
0xe2x89xa6cxe2x89xa61.0, and
n is a number determined by and consistent with the valence requirements of the other elements present in the compound oxide of formula (I),
which comprises the following steps (1) to (5):
(1) providing a solution or slurry of a raw material mixture in at least one liquid medium selected from the group consisting of water and an alcohol, the raw material mixture comprising a molybdenum (Mo) compound, a vanadium (V) compound, an antimony (Sb) compound and optionally at least one X compound selected from the group consisting of compounds of elements defined as X in formula (I),
(2) subjecting the raw material mixture solution or slurry to at least one oxidation treatment selected from the group consisting of:
(2-a) a heat treatment at 50 to 300xc2x0 C. in the presence of an oxidizing gas for 1 hour or more, and
(2-b) a treatment with an oxidizing liquid, thereby obtaining an oxidized raw material mixture solution or slurry,
(3) optionally adding to the oxidized raw material mixture solution or slurry obtained in step (2) at least one X compound selected from the group consisting of compounds of elements defined as X in formula (I),
(4) drying the oxidized raw material mixture solution or slurry obtained in step (2) or the oxidized raw material mixture solution or slurry having added thereto the X compound obtained in step (3), to thereby obtain a dried catalyst precursor, and
(5) calcining the dried catalyst precursor to thereby obtain an oxide catalyst comprising a compound oxide of formula (I).
For an easy understanding of the present invention, the essential features and various preferred embodiments of the present invention are enumerated below.
1. A process for producing an oxide catalyst for use in producing acrylonitrile or methacrylonitrile from propane or isobutane by ammoxidation in the gaseous phase, the oxide catalyst comprising a compound oxide represented by the following formula:
Mo0.1VaSbbXcOnxe2x80x83xe2x80x83(I)
xe2x80x83wherein:
X represents at least one element selected from the group consisting of niobium, tungsten, chromium, titanium, tantalum, zirconium, hafnium, manganese, rhenium, iron, ruthenium, cobalt,.rhodium, nickel, palladium, platinum, copper, silver, zinc, boron, gallium, indium, germanium, tin, tellurium, phosphorus, lead, bismuth, rare earth elements, and alkaline earth metals; and
a, b, c and n are, respectively, the atomic ratios of vanadium (V), antimony (Sb), X and oxygen (O), relative to molybdenum (Mo),
xe2x80x83wherein
0.1xe2x89xa6axe2x89xa61.0,
0.01xe2x89xa6bxe2x89xa60.6,
0xe2x89xa6cxe2x89xa61.0, and
n is a number determined by and consistent with the valence requirements of the other elements present in the compound oxide of formula (I),
which comprises the following steps (1) to (5):
(1) providing a solution or slurry of a raw material mixture in at least one liquid medium selected from the group consisting of water and an alcohol, the raw material mixture comprising a molybdenum (Mo) compound, a vanadium (V) compound, an antimony (Sb) compound and optionally at least one X compound selected from the group consisting of compounds of elements defined as X in formula (I),
(2) subjecting the raw material mixture solution or slurry to at least one oxidation treatment selected from the group consisting of:
(2-a) a heat treatment at 50 to 300xc2x0 C. in the presence of an oxidizing gas for 1 hour or more, and
(2-b) a treatment with an oxidizing liquid, thereby obtaining an oxidized raw material mixture solution or slurry,
(3) optionally adding to the oxidized raw material mixture solution or slurry obtained in step (2) at least one X compound selected from the group consisting of compounds of elements defined as X in formula (I),
(4) drying the oxidized raw material mixture solution or slurry obtained in step (2) or the oxidized raw material mixture solution or slurry having added thereto the X compound obtained in step (3), to thereby obtain a dried catalyst precursor; and
(5) calcining the dried catalyst precursor to thereby obtain.an oxide catalyst comprising a compound oxide of formula (I).
2. The process according to item 1 above, wherein the oxidizing gas is at least one member selected from the group consisting of oxygen, air and nitrogen oxides.
3. The process according to item 1 or 2 above, wherein the oxidizing liquid is an aqueous solution of at least one oxidizer compound selected from the group consisting of hydrogen peroxide, nitric acid and hypochlorous acid.
4. The process according to item 3 above, wherein the amount of the oxidizing liquid is in the range of from 0.01 to 2 in terms of the molar ratio of the at least one oxidizer compound to the antimony contained in the raw material mixture solution or slurry provided in step (1).
5. The process according to item 3 or 4 above, wherein the oxidizing liquid is an aqueous solution of hydrogen peroxide.
6. The process according to any one of items 1 to 5 above, wherein the calcination in step (5) is performed in an inert gas atmosphere which is substantially free from molecular oxygen.
7. The process according to any one of items 1 to 6 above, wherein the calcination in step (5) is performed at 400 to 700xc2x0 C.
8. The process according to any one of items 1 to 7 above, which further comprises the steps of (6) contacting the oxide catalyst obtained in step (5) with an aqueous basic solution to obtain a contact mixture containing a base-treated oxide catalyst, and (7) separating and recovering the base-treated oxide catalyst from the contact mixture.
9. The process according to item 8 above, wherein the aqueous basic solution is an aqueous solution of at least one compound selected from the group consisting of ammonia, an alkali metal hydroxide, an alkaline earth metal hydroxide, an alkali metal carbonate, an alkaline earth metal carbonate, an alkali metal salt of an organic acid, an alkaline earth metal salt of an organic acid and an amine.
10. The process according to item 9 above, wherein the aqueous basic""solution is an aqueous solution of ammonia.
11. The process according to any one of items 8 to 10 above, which further comprises the step (8) of calcining the base-treated oxide catalyst recovered in step (7) from the contact mixture.
12. The process according to item 11 above, wherein the calcination in step (8) is performed in an inert gas atmosphere which is substantially free from molecular oxygen.
13. The process according to item 11 or 12 above, wherein the calcination in step (8) is performed at 400 to 700xc2x0 C.
14. An oxide catalyst for use in producing acrylonitrile or methacrylonitrile from propane or isobutane by ammoxidation in the gaseous phase, which is substantially the same as that produced by the process of any one of items 1 to 13 above.
15. A process for producing acrylonitrile or methacrylonitrile, which comprises reacting propane or isobutane with ammonia and molecular oxygen in the gaseous phase in the presence of the oxide catalyst of item 14 above.
Hereinbelow, the present invention will be described in more detail.
The compound oxide of the oxide catalyst produced by the process of the present invention has a composition represented by the following formula (I):
M0.1VaSbbXcOnxe2x80x83xe2x80x83(I)
wherein:
X represents at least one element selected from the group consisting of niobium, tungsten, chromium, titanium, tantalum, zirconium, hafnium, manganese, rhenium, iron, ruthenium, cobalt, rhodium, nickel, palladium, platinum, copper, silver, zinc, boron, gallium, indium, germanium, tin, tellurium, phosphorus, lead, bismuth, rare earth elements, and alkaline earth metals; and
a, b, c and n are, respectively, the atomic ratios of vanadium (V), antimony (Sb), X and oxygen (O), relative to molybdenum (Mo),
xe2x80x83wherein
0.1xe2x89xa6axe2x89xa61.0,
0.01xe2x89xa6bxe2x89xa60.6,
0xe2x89xa6cxe2x89xa61.0, and
n is a number determined by and consistent with the valence requirements of the other elements present in the compound oxide of formula (I).
As element X in formula (I) above, preferred are niobium (Nb), tungsten (W), tin (Sn) and titanium (Ti), and especially preferred are Nb and Ti. It is preferred that the atomic ratio (a) of vanadium (V) to Mo satisfies the relationship: 0.1xe2x89xa6axe2x89xa60.5, more preferably 0.2xe2x89xa6axe2x89xa60.4. It is preferred that the atomic ratio (b) of antimony (Sb) to Mo satisfies the relationship: 0.1xe2x89xa6bxe2x89xa60.3, more preferably 0.1xe2x89xa6bxe2x89xa60.25. It is preferred that the atomic ratio (c) of X to Mo satisfies the relationship: 0.01xe2x89xa6cxe2x89xa60.5, more preferably 0.01xe2x89xa6cxe2x89xa60.2, with the proviso that, when the above-mentioned compound oxide contains tellurium (Te) as element X, it is preferred that the atomic ratio (cxe2x80x2) of Te to Mo generally satisfies the relationship: 0.001xe2x89xa6cxe2x80x2xe2x89xa60.07, preferably 0.001xe2x89xa6cxe2x80x2xe2x89xa60.03, especially preferably 0.001xe2x89xa6cxe2x80x2xe2x89xa60.01.
Conventionally, an oxide catalyst containing Mo, V and Sb is produced by a process comprising:
preparing a solution or slurry of a raw material mixture comprising a molybdenum (Mo) compound, a vanadium (V) compound and an antimony (Sb) compound;
drying the raw material mixture solution or slurry, thereby obtaining a dried catalyst precursor (drying step); and
calcining the obtained dried catalyst precursor (calcination step).
In contrast, the essential feature of the process of the present invention resides in that, in the process for producing an oxide catalyst comprising a compound oxide represented by formula (I) above which contains at least Mo, V and Sb, a solution or slurry, in water and/or an alcohol, of a raw material mixture comprising at least an Mo compound, a V compound and an Sb compound is subjected to a specific oxidation treatment as described below prior to the drying step and the subsequent calcination step, thereby remarkably advancing the oxidation of the Mo, V and Sb. By virtue of such a characteristic feature of the process of the present invention, it has become possible to produce an oxide catalyst which is not only insusceptible to deterioration during the production of the (meth)acrylonitrile, but also useful for effectively, efficiently producing (meth)acrylonitrile in high yield and high space time yield.
As mentioned above, the process of the present invention comprises the following steps (1) to (5):
(1) providing a solution or slurry, in water and/or an alcohol, of a raw material mixture comprising a molybdenum compound, a vanadium compound, an antimony compound and optionally an X compound,
(2) subjecting the raw material mixture solution or slurry to a specific oxidation treatment, thereby obtaining an oxidized raw material mixture solution or slurry,
(3) optionally adding to the oxidized raw material mixture solution or slurry obtained in step (2) an X compound,
(4) drying the oxidized raw material mixture solution or slurry obtained in step (2) or (3) which optionally contains the X compound, to thereby obtain a dried catalyst precursor, and
(5) calcining the dried catalyst precursor to thereby obtain an oxide catalyst comprising a compound oxide of formula (I).
Hereinbelow, the steps involved in the process of the present invention will be described in more detail.
Step (1). Raw Material Mixture Preparation Step
In step (1) of the process of the present invention, a raw material mixture comprising a molybdenum compound, a vanadium compound, an antimony compound and optionally at least one X compound selected from the group consisting of compounds of elements defined as X in formula (I) is dissolved or dispersed in at least one liquid medium selected from the group consisting of water and an alcohol so as to provide a solution or slurry of the raw material mixture.
As a liquid medium, at least one medium selected from, the group consisting of water and an alcohol can be used. Examples of alcohols used in the present invention include C1-C4 alcohols and benzyl alcohol. With respect to the amount of the liquid medium, there is no particular limitation as long as the amount of the liquid medium is sufficient for forming a solution or slurry of the raw material mixture. However, from the viewpoint of the solubility of the respective raw material compounds of the component elements and the promotion of the reactions subsequently occurring in the liquid medium; it is preferred to use 4 to 20 g of the liquid medium per gram of the molybdenum compound in the raw material mixture. When a mixture of water and an alcohol is used as a liquid medium, the ratio of water to the alcohol is appropriately selected, taking into consideration the solubility of the compounds contained in the raw material mixture in water and the alcohol.
In the process of the present invention for producing an oxide catalyst comprising a compound oxide, the following compounds can be used as sources of the component elements of the oxide catalyst.
Examples of sources of molybdenum include ammonium heptamolybdate, molybdenum oxides, molybdenum oxychlorides, molybdenum alkoxides and the like. Of these, hexavalent molybdenum compounds, especially ammonium heptamolybdate is preferred.
Examples of sources of vanadium include ammonium metavanadate, vanadium (V) oxide, vanadium oxychlorides, vanadium alkoxides and the like. Of these, pentavalent vanadium compounds, especially ammonium metavanadate or vanadium (V) oxide is preferred.
Examples of sources of antimony include antimony (III) oxide, antimony (V) oxide, antimony chlorides, antimony (III) oxychloride, antimony (III) nitrate oxide, antimony alkoxides, and organic acid salts of antimony, such as antimony tartrate and the like. Of these, trivalent antimony compounds, especially antimony (III) oxide is preferred.
Examples of sources of X elements include dicarboxylic acid salts, hydroxides, oxides, nitrates, acetates, ammonium salts, carbonates and alkoxides of the X elements and the like. When niobium and/or titanium is used as the X element, an aqueous solution of a dicarboxylate thereof can be preferably used. With respect to the source of niobium, it is preferred to use a niobic acid. The xe2x80x9cniobic acidxe2x80x9d is a hydrated compound represented by the following formula: Nb2O5.nH2O, which is also known as xe2x80x9cniobium hydroxidexe2x80x9d or xe2x80x9cniobium oxide hydratexe2x80x9d. It is especially preferred to use a niobium-containing aqueous solution disclosed in EP 0895809 A1, which comprises water having dissolved therein a dicarboxylic acid, a niobium compound and optionally ammonia, wherein the molar ratio of the dicarboxylic acid to niobium is in the range of from 1 to 4, and the ammonia/niobium molar ratio is 0 to 2. In the present invention, however, the molar ratio of the dicarboxylic acid to niobium can be more than 4, as long as the molar ratio is 10 or less. In the niobium-containing aqueous solution used in the present invention, the preferred molar ratio of the dicarboxylic acid to niobium is in the range of 2 to 6. As the above-mentioned dicarboxylic acid, preferred is oxalic acid.
The suitable amounts of the molybdenum compound, vanadium compound, antimony compound and X-element compound as a compound of an optional component element vary depending on the types of the compounds used, and the amounts are appropriately selected so that a compound oxide having the composition represented by formula (I) is obtained.
There is no particular limitation with respect to the method for preparing a raw material mixture solution or slurry by combining the above-mentioned liquid medium, molybdenum compound, vanadium compound, antimony compound and X element compound as a compound of an optional component element. As specific examples of methods for preparing a raw material mixture solution or slurry, there can be mentioned the following methods. An aqueous solution or slurry containing a binary mixture of ammonium metavanadate and antimony (III) oxide is heated in an atmosphere of air so as to effect a reaction. It is preferred that the aqueous solution or slurry is heated to a temperature of from 70 to 110xc2x0 C. Heating of the aqueous solution or slurry may be conducted while adding water to the solution or slurry to make up for the water lost by evaporation, or conducted under reflux conditions using a reactor equipped with a condenser. It is preferred that the time for heating is 3 hours or more, especially from 4 to 50 hours. As a source of vanadium, in place of ammonium metavanadate, a solution of vanadium (V) oxide in an aqueous solution of hydrogen peroxide or an aqueous solution of the above-mentioned sources of vanadium other than ammonium metavanadate and vanadium (V) oxide also.can be used. To the aqueous solution or slurry containing the binary mixture of a vanadium compound and an antimony compound, which is obtained by the above-mentioned method, ammonium heptamolybdate or an aqueous solution thereof may be added, thereby obtaining an aqueous solution or slurry of the raw material mixture comprising a molybdenum compound, a vanadium compound and an antimony compound.
Alternatively, the raw material mixture solution or slurry can be prepared by another method described below. An aqueous solution or slurry containing a binary mixture of ammonium heptamolybdenum and antimony (III) oxide is heated so as to effect a reaction to obtain a reaction mixture, followed by addition of a vanadium compound or an aqueous solution thereof to the reaction mixture, thereby obtaining an aqueous solution or slurry of the raw material mixture comprising a molybdenum compound, a vanadium compound and an antimony compound.
When it is intended to provide an aqueous solution or slurry of a raw material mixture containing an X element compound as a compound of an optional component element, there is no particular limitation with respect to the timing to combine an X element compound with other compounds of the raw material mixture. For example, the X element compound can be added to the above-mentioned aqueous solution or slurry containing the binary mixture of a vanadium compound and an antimony compound or the binary mixture of a molybdenum compound and an antimony compound (e.g., a mixture of ammonium heptamolybdenum and antimony (III) oxide), or to the above-mentioned aqueous solution or slurry of the raw material mixture comprising a molybdenum compound, a vanadium compound and an antimony compound.
When alkoxides are used as sources of molybdenum, vanadium and/or antimony, if water is used as a liquid medium, the alkoxides are disadvantageously hydrolyzed. Therefore, when alkoxides are used as sources of molybdenum, vanadium and/or antimony, it is preferred that an alcohol is used as a liquid medium.
Step (2). Oxidation Treatment Step
In step (2) of the present invention, the raw material mixture solution or slurry prepared by the above-mentioned method is subjected to at least one oxidation treatment selected from the group consisting of (2-a) a heat treatment at 50 to 300xc2x0 C. in the presence of an oxidizing gas for 1 hour or more, and (2-b) a treatment with an oxidizing liquid, thereby obtaining an oxidized raw material mixture solution or slurry.
First, an explanation is made below with respect to oxidation treatment (2-a).
There is no particular limitation with respect to the oxidizing gas. However, it is preferred to use at least one member selected from the group consisting of oxygen, air and nitrogen oxides. Nitrogen oxides mean nitrous oxide, nitrogen monoxide, nitrogen dioxide and the like. As an oxidizing gas, preferred is oxygen or air. The air may be in the form of an oxygen-rich air.
The temperature for heat treatment in oxidation treatment (2-a) is in the range of from 50 to 300xc2x0 C., preferably from 50 to 200xc2x0 C., more preferably from 70 to 110xc2x0 C. The time for heat treatment is 1 hour or more, preferably 3 hours or more, more preferably from 4 to 15 hours.
It is preferred that heat (oxidation) treatment (2-a) of the solution or slurry of the raw material mixture in the presence of the oxidizing gas is conducted using an apparatus equipped with a reflux condenser, or an autoclave or the like under conditions wherein the drying of the raw material mixture due to the evaporation of the liquid medium will not occur.
For example, heat treatment (2-a) can be conducted by the following method. The solution or slurry of the raw material mixture is subjected to the oxidation treatment in an apparatus equipped with a reflux condenser by heating at 70 to 110xc2x0 C. in the presence of the oxidizing gas. It is preferred that the solution or slurry of the raw material mixture is stirred in an atmosphere of an oxidizing gas so as to increase the area of the interface between the solution or slurry of the raw material mixture and the oxidizing gas, thereby promoting the oxidation. It is more preferred that the oxidizing gas is blown into the solution or slurry of the raw material mixture. When a reflux condenser is not used, it is preferred that an appropriate amount of water is added so that the drying of the raw material mixture does not occur. When an autoclave is used for heat treatment (2-a), heat treatment (2-a) can be conducted by a method comprising adding the solution or slurry of the raw material mixture to the autoclave, introducing the oxidizing gas into the autoclave, and subjecting the solution or slurry of the raw material mixture to the oxidation treatment at 100 to 300xc2x0 C. or more. The temperature is preferably from 100 to 150xc2x0 C. When an alcohol is used as a liquid medium, the heating can be conducted at 50 to 150xc2x0 C. by means of an apparatus equipped with a reflux condenser. The time for heat treatment is preferably 2 hours or more, more preferably from 4 to 15 hours.
Next, explanation is made with respect to oxidation treatment (2-b).
There is no particular limitation with respect to the oxidizing liquid. However, it is preferred that the oxidizing liquid is an aqueous solution of at least one oxidizer compound selected from the group consisting of hydrogen peroxide, nitric acid and hypochlorous acid. Of these, preferred is an aqueous solution of hydrogen peroxide (i.e., aqueous hydrogen peroxide). There is no particular limitation with respect to the amount of the oxidizing liquid used. However, the molar ratio of the oxidizer compound contained in the oxidizing liquid to antimony contained in the solution or slurry of the raw material mixture is preferably 0.01 to 2, more preferably 0.1 to 1.
After the addition of the oxidizing liquid to the solution or slurry of the raw material mixture, the resultant mixture may be stirred while cooling at a temperature wherein the liquid medium is not solidified (for example, when water is used as the liquid medium, at 0 to 20xc2x0 C.), at a temperature around room temperature (for example, from 20 to 30xc2x0 C.) or while heating at a temperature of 100xc2x0 C. or lower. When the mixture is heated to a temperature higher than the boiling point of the liquid medium, the oxidizing treatment can be conducted by means of an autoclave. The time for the oxidizing treatment is preferably 30 minutes or more, more preferably from 1 to 2 hours.
It is preferred that the solution or slurry of the oxidized raw material mixture obtained by oxidation treatment of the above-mentioned methods for (2-a) and/or (2-b) is maintained in an atmosphere of an inert gas.
With respect to the process for preparing the oxide catalyst described in the above-mentioned Unexamined Japanese Patent Application Laid-Open Specification Nos. 9-157241 and 10-28862, it is possible that, when the catalyst is produced in an atmosphere of air, in the step for the preparation of the solution or slurry of the raw material mixture comprising a Mo compound, a V compound, a Sb compound and an optionally Nb compound, the oxidation reaction may occur to some extent. However, in the above documents, any specific treatment for enhancing the oxidation state of Mo, V and Sb was not conducted. On the other hand, in the present invention, the oxidation state of Mo, V and Sb contained in the solution or slurry of the raw material mixture is extremely enhanced by subjecting the solution or slurry comprising at least Mo compound, V compound and Sb compound to the above-mentioned specific oxidation treatment. By this treatment, it becomes possible to obtain an oxide catalyst which is not only almost insusceptible to deterioration during the production of (meth)acrylonitrile, but also effective for producing (meth)acrylonitrile in high yield and high space time yield.
Step (3). X Element Compound (as a Compound of an Optional Component Element) Addition Step
In step (3), an X element compound as a compound of an optional component element can be optionally added to the solution or slurry of the oxidized raw material mixture obtained in step (2). When the solution or slurry of the raw material mixture containing an unsatisfactory amount of an X element compound is prepared in step (1), an appropriate amount of the X element compound is supplementarily added, in step (3), to the solution or slurry of the oxidized raw material mixture obtained in step (2) prior to the below-mentioned drying process. When the solution or slurry of the raw material mixture provided in the above-mentioned step (1) contains an X element compound, the X element compound may or may not be oxidized during the above-mentioned oxidation treatment step, depending on the type of the X element compound. However, whether or not the X element compound is oxidized has no influence on the effects of the present invention.
Step (4). Drying Step
In step (4) of the process of the present invention, the above-mentioned solution or slurry of the oxidized raw material mixture is dried so as to obtain a dried catalyst precursor.
The drying of the solution or slurry of the oxidized raw material mixture can be conducted by a known method, such as spray drying or evaporation drying. The spray drying can be conducted, for example, by spraying and heating the solution or slurry of the oxidized raw material mixture in the dryer. The spray drying can be conducted by centrifugation, by the two-phase flow nozzle method or by the high pressure nozzle method. As a heat source for drying, it is preferred to use air which has been heated by steam, an electric heater and the like. It is preferred that the temperature of the dryer at an entrance to the dryer section thereof is from 150 to 300xc2x0 C. The spray drying can be also conveniently conducted by spraying the solution or slurry of the oxidized raw material mixture onto an iron plate which has been heated to a temperature of 100 to 300xc2x0 C.
The solution or slurry of the oxidized raw material mixture can be evaporation-dried by heating it in a vessel, such as a beaker, at 100 to 300xc2x0 C. The time for heating varies depending on the composition or amount of the solution or slurry of the oxidized raw material, but is generally from 5 minutes to 20 hours, preferably from 5 minutes to 3 hours.
The dried catalyst precursor can be obtained in a powder form by the above-mentioned drying step.
Step (5). Calcination Step
In step (5) of the process of the present invention, the above-mentioned dried catalyst precursor is calcined so as to obtain an oxide catalyst comprising component oxide represented by formula (1) above.
The calcination can be conducted by a known method. For example, use can be made of a kiln, such as a rotary kiln, a tunnel kiln, a muffle kiln or a fluidized firing kiln. The calcination is generally conducted in an atmosphere of an inert gas, such as nitrogen gas under atmospheric pressure, which is substantially free of oxygen, preferably under a flow of an inert gas, at a temperature of 400 to 700xc2x0 C., preferably 500 to 700xc2x0 C., more preferably 550 to 650xc2x0 C. The time of calcination is generally 0.5 to 5 hours, preferably 1 to 3 hours. It is preferred that the oxygen concentration in the above-mentioned inert gas is 1000 ppm or less, more preferably 100 ppm or less as measured by gas chromatography or by means of a trace oxygen analyzer. The calcination can be conducted repeatedly. Prior to the calcination, the dried catalyst precursor may be subjected to pre-calcination in an atmosphere of air or under a stream of air at 200 to 420xc2x0 C., preferably 250 to 350xc2x0 C. for 10 minutes to 5 hours. The catalyst obtained by calcination may be subjected to further calcination in an atmosphere of air at a temperature of from 200 to 400xc2x0 C. for 5 minutes to 5 hours. The catalyst obtained by calcination may be pulverized and subjected to further calcination.
When it is intended to produce a silica-supported catalyst, the addition of a silica sol can be made at an any time in the above-mentioned steps (1) and (2). The amount of silica is generally in the range of from 20 to 60% by weight, preferably 20 to 40% by weight, based on the total weight of the catalyst and silica. It is preferred to use a silica sol stabilized by ammonium ions to prevent gelation. It is preferred that the silica sol contains ammonium ions in an amount sufficient for keeping the pH of the silica sol at around 9.7.
The oxide catalyst obtained by the process of the present invention which comprises the above-mentioned steps (1) to (5), as such, can be advantageously used for producing (meth)acrylonitrile in high yield and in high space time yield. However, when use is made of a base-treated oxide catalyst obtained by the process according to another aspect of the present invention which further comprises the steps of (6) contacting the above-mentioned oxide catalyst with an aqueous basic solution to obtain a contact mixture containing a base-treated oxide catalyst, and (7) separating and recovering said base-treated oxide catalyst from the contact mixture, it has become possible to produce (meth)acrylonitrile in further improved yield and space time yield. In this connection, it should-be noted that, by this embodiment, it has also become possible to suppress the decomposition of ammonia, which is one of the materials for the production of (meth)acrylonitrile, into nitrogen. The reason why such excellent effects can be obtained has not yet been elucidated. However, it is considered to be as follows: The oxide catalyst obtained in step (5) contains undesired, noneffective phases of component oxides, such as a non-crystalline molybdenum oxide, and these undesired, noneffective phases of component oxides induce the burning or decomposition of ammonia or (meth)acrylonitrile. However, by the treatment with an aqueous basic solution, the above-mentioned undesired, noneffective phases of component oxides can be removed, so that not only are the yield and space time yield of (meth)acrylonitrile improved, but also the decomposition of ammonia into nitrogen is suppressed.
Hereinbelow, explanation is made with respect to the above-mentioned steps (6) and (7) for the treatment with an aqueous basic solution.
In step (6), the oxide catalyst is contacted with the aqueous basic solution so as to obtain a contact mixture comprising a base-treated oxide catalyst.
Examples of basic substances contained in the aqueous basic solution include ammonia; hydroxides, carbonates and organic acid salts (for example, oxalates, acetates and citrates) of alkali metals, such as lithium, sodium, potassium, rubidium, cesium and the like; hydroxides, carbonates and organic acid salts (for example, oxalates, acetates and citrates) of alkaline earth metals, such as beryllium, magnesium, calcium, strontium, barium and the like; and amines, such as ethylamine, methylamine and the like. Especially preferred is aqueous ammonia. The concentration of the basic substance in the aqueous basic solution is generally in the range of from 0.1 to 30% by weight, preferably from 1 to 10% by weight.
As examples of the methods for contacting the oxide catalyst with the aqueous basic solution, there can be mentioned a batchwise operation method and a flow operation method.
When the batchwise operation method is employed, the oxide catalyst is suspended in the aqueous basic solution in a tank reactor so as to obtain a contact mixture in the form of a suspension.
The weight ratio of the catalyst to the aqueous basic solution is preferably in the range of from 1 to 20% by weight. There is no particular limitation with respect to the period during which the oxide catalyst is in contact with the aqueous basic solution. However, preferred is 2 hours or more, more preferred is 6 hours or more. It is preferred that the temperature of the aqueous basic solution is in the range of from 0 to 100xc2x0 C. When aqueous ammonia is used as the aqueous basic solution, it is more preferred that the temperature of the aqueous basic solution is in the range of from 0 to 50xc2x0 C. For effectively performing an intimate contact of the oxide catalyst with the aqueous basic solution, it is preferred to stir the contact mixture by means of a stirrer or blow a gas into the contact mixture. There is no particular limitation with respect to the above-mentioned gas. However, it is preferred to use a gas which would not change the aqueous basic solution to a neutral solution. Examples of such gases include air, nitrogen gas and a rare gas. As an example of the flow operation method, there can be mentioned a method in which the aqueous basic solution is flowed through a fixed bed formed by charging the oxide catalyst in a reactor.
In step (7), the base-treated oxide catalyst is separated and recovered from the contact mixture. The separation can be conducted by centrifugation, filtration using a filter paper or a filter membrane, decantation and the like. It is preferred that the oxide catalyst separated and recovered from the above-mentioned contact mixture is washed with water, followed by drying. The drying after the above-mentioned treatment with the aqueous basic solution can be conducted, using a heated air dryer, a constant temperature chamber or the like, at 100 to 150xc2x0 C.
Further, in the process of the present invention, as step (8), it is preferred to calcine the base-treated oxide catalyst recovered in the above-mentioned step (7). The calcination after the treatment with the aqueous basic solution can be performed under the same conditions as in the calcination of the above-mentioned step (5). That is, the calcination can be, conducted in an atmosphere of an inert gas, such as nitrogen gas or the like, which is substantially free of oxygen, preferably under a flow of an inert gas, at 400 to 700xc2x0 C., preferably 500 to 700xc2x0 C., more preferably 550 to 650xc2x0 C.
The operation comprising the treatment using the aqueous basic solution in the above-mentioned steps (6) and (7) and subsequent calcination in the above-mentioned step (8) can be repeatedly conducted so as to further improve the yield and space time yield of (meth)acrylonitrile.
Hereinbelow, explanation is made with respect to the process, for producing (meth)acrylonitrile from propane or isobutane by ammoxidation in the gaseous phase in the presence of the oxide catalyst comprising the compound oxide prepared by the process of the present invention.
The feedstock mixture comprising propane or isobutane and ammonia need not be of a very high purity but may be of a commercial grade.
Examples of sources of oxygen include air, oxygen-rich air, and pure oxygen. Further, such a source of molecular oxygen may be diluted with helium, argon, nitrogen, carbon dioxide, steam or the like.
The molar ratio of ammonia to propane or isobutane for the ammoxidation may be generally in the range of from 0.1 to 1.5, preferably from 0.2 to 1.2.
The molar ratio of molecular oxygen to propane or isobutane used for the ammoxidation may be generally in the range of from 0.2 to 6, preferably from 0.4 to 4.
The ammoxidation pressure is generally in the range of from 0.1 to 10 atm, preferably from 1 to 3 atm.
The ammoxidation temperature is generally in the range of from 350 to 600xc2x0 C., preferably from 380 to 470xc2x0 C.
The time of contact (contact time) between the gaseous feedstocks and the catalyst is generally in the range of from 0.1 to 30 (gxc2x7sec/ml), preferably from 0.5 to 10 (gxc2x7sec/ml).
In the process of the present invention, the contact time is determined according to the following formula:
Contact time(gxc2x7sec/ml)       Contact    ⁢          xe2x80x83        ⁢    time    ⁢          xe2x80x83        ⁢          (              g        ·                  sec          /          ml                    )        =            (              W        /        F            )        xc3x97    60    xc3x97          273              (                  273          +          T                )              xc3x97    P  
wherein:
W represents the weight (g) of the catalyst contained in the reactor;
F represents the flow rate (ml/s) of the raw material mixture gas {as measured under normal temperature and pressure conditions (0xc2x0 C., 1 atm)};
T represents the ammoxidation reaction temperature (xc2x0 C.); and
P represents the ammoxidation reaction pressure (atm).
The reaction of the present invention can be conducted in a conventional reactor, such as a fixed bed reactor, a fluidized-bed reactor or a moving bed reactor.
The reaction mode employed in the process of the present invention may be either a one pass mode or a recycling mode.
Hereinbelow, the present invention will be described in more detail with reference to the following Examples and Comparative Examples, which are for illustrative purposes only, and should not be construed as limiting the scope of the present invention.